Retroreflective films are widely used for highway and personal safety products. Their primary function is to reflect a high percentage of light projected from a light source back in the general direction of the light source. The brightness, i.e., percentage of light reflected, and color, e.g., whiteness, of these materials are important factors and frequently part of the specification. For example, the Department of Transportation (DOT) sets rules governing the use of retroreflective films on tractor trailers ambulances highway safety cones and barrels. DOT standards often require these materials to be produced in colors other than white which are highly visible during the day. For example, bright orange or yellow are used on ambulances, as well as on highway safety cones and barrels.
Many conventional materials use a construction of glass beads, which provide substantial retroreflectivity, deposited in a pattern on a white substrate to permit a white background to be present in about 30% of the area. Other conventional materials use a prismatic or corner-cube structure to achieve a high retroreflectivity. Rowland has disclosed in U.S. Pat. No. 5,229,882, issued Jul. 20, 1993, a structure having improved properties of retroreflectivity and whiteness. However the disclosed film is costly and difficult to produce in large quantities.
Conventionally, a plastic film of polyvinyl chloride (PVC) or polyethylene teraphthalate (PET) is embossed using any one of a variety of methods known in the art. The embossed pattern creates on a surface of the film a plurality of corner-cubes or prisms, each of which reflects light projected from a light source back to the light source. Such patterns are well known in the art of light reflective bodies and materials. In some conventional materials, a layer of metal is vacuum deposited onto the embossed surface of the film to improve the reflectivity. However, the metallization layer also changes the color of the resultant material. Usually metallization darkens the color. Rowland suggests the metallization be vapor deposited in a pattern in order to improve the color and brightness of the final product. Areas having metal deposited thereon are highly reflective while in other areas where metal is absent, the substrate color predominates. However, in practice it is very difficult to apply precise patterns of metal to the prisms either directly using a pattern metallized printing process or using a traditional caustic removal process. The prisms interfere with either the caustic application or the pattern metallization process. For example, caustic may collect in recesses of the prism structure, resulting in excess removal of material in some areas.
Rowland further teaches enhancing the photometric properties of non-metallized prisms by adhering another film to the metallized surface of the prism embossed film using a thin layer of an adhesive which is white in color. This process is costly and the thinness of the adhesive makes it difficult to obtain high levels of whiteness and brightness.